1. Field of the Invention
The present invention relates to a lighting controller of a lighting device for a vehicle and, more particularly, to a lighting controller of a lighting device for a vehicle that serves to control a lighting operation of a semiconductor light source constituted by a semiconductor light emitting device.
2. Related Art
Conventionally, there has been known a lighting device for a vehicle that uses, as a semiconductor light source, a semiconductor light emitting device, for example, a light emitting diode (LED). A lighting controller for controlling a lighting operation of the LED is mounted on the lighting device for a vehicle of this type.
The lighting controller is constituted by connecting a single switching regulator to a plurality of series regulators (for example, see Patent Document 1).
The single switching regulator includes a transformer, a capacitor, a diode, and an NMOS (Negative Channel Metal Oxide Semiconductor) transistor. The single switching regulator functions as current supplying means for supplying a driving current to a plurality of LEDs.
A plurality of series regulators includes an NMOS transistor, a shunt resistor, and a comparison amplifier respectively, and carries out the ON/OFF operations and dimming of the LED through a constant current control.
The shunt resistor detects a driving current (an LED driving current) supplied from the single switching regulator to the LED as a voltage generated on both ends of the shunt resistor (which will be hereinafter referred to as a “detected voltage”). The detected voltage is applied to an inverted input terminal (a negative input terminal) of the comparison amplifier.
The comparison amplifier compares the detected voltage applied to the negative input terminal with a reference voltage applied to a non-inverted input terminal (a positive input terminal) and applies a voltage (a comparing output) corresponding to a result of the comparison to a gate of the NMOS transistor to control ON/OFF operations of the NMOS transistor.
The comparing output applied to the NMOS transistor is fed back to a control circuit. The control circuit decides whether the operation of the single switching regulator is stopped or not upon receipt of the comparing output, which is fed back, and controls the NMOS transistor constituting the switching regulator in order to stop the operation of the single switching regulator when the comparing output exceeds a predetermined threshold.
When an output of the LED is opened (a first abnormal state), a lower voltage than the reference voltage of the positive input terminal is applied to the negative input terminal of the comparison amplifier. Therefore, the comparing output is sent from the comparison amplifier having a greater value than before. As a result, the first abnormal state is detected by the control circuit monitoring a connecting node voltage between the comparison amplifier and the NMOS transistor.
When an anode and a cathode in the LED are short-circuited (a second abnormal state), a voltage on the anode side is dropped so that a voltage on the output side of the switching regulator is dropped. The second abnormal state is detected by the control circuit monitoring the voltage on the output side of the switching regulator.
When the anode side of the LED is grounded (a third abnormal state), the voltage on the anode side is reduced so that the voltage on the output side of the switching regulator is dropped in the same manner as the second abnormal state. The third abnormal state is detected by the control circuit monitoring the voltage on the output side of the switching regulator.
When the cathode side of the LED is grounded (a fourth abnormal state), a consistency of the number of the LEDs and a current supplied to each of the LEDs is monitored by a current detector provided on the output side of the switching regulator and it is decided that the fourth abnormal state is brought in case of an inconsistency. For example, if the cathode is grounded in only one of the LEDs, a current value of the LED driving current supplied to all of the LEDs is greater than that of the LED driving current supplied to all of the LEDs in a normal state. When it is detected that a total value of the LED driving currents is greater than a total value of the LED driving currents in the normal case, the control circuit recognizes that any of the LEDs is brought into the fourth abnormal state.
[Patent Document 1] JP-A-2006-103477 Publication